Key senators say Congress has outlawed one of the most notorious detainee interrogation techniques -- "waterboarding," in which a prisoner feels near drowning. But the White House will not go that far, saying it would be wrong to tell terrorists which practices they might face.

Inside the CIA, waterboarding is cited as the technique that got Khalid Sheik Mohammed, the prime plotter of the Sept. 11, 2001, terrorist attacks, to begin to talk and provide information -- though "not all of it reliable," a former senior intelligence official said.

Waterboarding is variously characterized as a powerful tool and a symbol of excess in the nation's fight against terrorists. But just what is waterboarding, and where does it fit in the arsenal of coercive interrogation techniques?

On Jan. 21, 1968, The Washington Post published a front-page photograph of a U.S. soldier supervising the questioning of a captured North Vietnamese soldier who is being held down as water was poured on his face while his nose and mouth were covered by a cloth. The picture, taken four days earlier near Da Nang, had a caption that said the technique induced "a flooding sense of suffocation and drowning, meant to make him talk."

The article said the practice was "fairly common" in part because "those who practice it say it combines the advantages of being unpleasant enough to make people talk while still not causing permanent injury."

The picture reportedly led to an Army investigation.

Twenty-one years earlier, in 1947, the United States charged a Japanese officer, Yukio Asano, with war crimes for carrying out another form of waterboarding on a U.S. civilian. The subject was strapped on a stretcher that was tilted so that his feet were in the air and head near the floor, and small amounts of water were poured over his face, leaving him gasping for air until he agreed to talk.

"Asano was sentenced to 15 years of hard labor," Sen. Edward M. Kennedy (D-Mass.) told his colleagues last Thursday during the debate on military commissions legislation. "We punished people with 15 years of hard labor when waterboarding was used against Americans in World War II," he said.

A CIA interrogation training manual declassified 12 years ago, "KUBARK Counterintelligence Interrogation -- July 1963," outlined a procedure similar to waterboarding. Subjects were suspended in tanks of water wearing blackout masks that allowed for breathing. Within hours, the subjects felt tension and so-called environmental anxiety. "Providing relief for growing discomfort, the questioner assumes a benevolent role," the manual states.

The KUBARK manual was the product of more than a decade of research and testing, refining lessons learned from the Korean War, where U.S. airmen were subjected to a new type of "touchless torture" until they confessed to a bogus plan to use biological weapons against the North Koreans.

Used to train new interrogators, the handbook presented "basic information about coercive techniques available for use in the interrogation situation." When it comes to torture, however, the handbook advised that "the threat to inflict pain . . . can trigger fears more damaging than the immediate sensation of pain."

In the post-Vietnam period, the Navy SEALs and some Army Special Forces used a form of waterboarding with trainees to prepare them to resist interrogation if captured. The waterboarding proved so successful in breaking their will, says one former Navy captain familiar with the practice, "they stopped using it because it hurt morale."

After the Sept. 11, 2001, terror attacks, the interrogation world changed. Low-level Taliban and Arab fighters captured in Afghanistan provided little information, the former intelligence official said. When higher-level al-Qaeda operatives were captured, CIA interrogators sought authority to use more coercive methods.

These were cleared not only at the White House but also by the Justice Department and briefed to senior congressional officials, according to a statement released last month by the Office of the Director of National Intelligence. Waterboarding was one of the approved techniques.

When questions began to be raised last year about the handling of high-level detainees and Congress passed legislation barring torture, the handful of CIA interrogators and senior officials who authorized their actions became concerned that they might lose government support.

Passage last month of military commissions legislation provided retroactive legal protection to those who carried out waterboarding and other coercive interrogation techniques.